The coal-bed gas reserve in China is very rich, but has not been exploited commercially at a large scale, mainly because that the coal-bed gas reservoirs are highly sensitive and may be easily contaminated by external fluids and solid phases in the drilling process, consequently the coal-bed gas seepage fractures and channels are clogged, the desorption of coal-bed gas is degraded, and the gas output is severely reduced.
At present, foam drilling fluid technique and under-balanced drilling technique have not been popularized widely yet; instead, a solid-free brine drilling fluid technique may be used for coal-bed gas wells drilling, i.e., the solid-free brine drilling fluid is a brine drilling fluid obtained by adding salts (e.g., KCl, NaCl, etc.) for weighting, instead of using bentonite, barite, and other solid particles. However, a solid-free brine drilling fluid that is highly adaptive to coal-bed gas wells is not available yet.